The present invention relates to a semiconductor device lead frame, a process of production of the same, a semiconductor device using the lead frame, and a process of production of the same.
A semiconductor device lead frame used in a resin sealing type semiconductor device is made using, other than an iron-nickel-based alloy (representatively Fe-42% Ni alloy), a copper-based material made of copper or a copper alloy.
In the case of a lead frame using this copper-based material, in order to flatten the surface and improve adhesion of a silver or other precious metal plating film to be formed later, generally, a copper strike plating film is formed on the surface of the material, then the above precious metal plating film for improving die bonding and wire bonding is formed on a die pad and on wire bonding portions of inner leads.
When using a copper-based material, due to exposure to a high temperature of about 150xc2x0 C. to 350xc2x0 C. in the step of mounting a semiconductor chip on the lead frame to assemble a semiconductor device, there is a problem that an oxide film of copper is apt to be formed and therefore the adhesion between the sealing resin and the lead frame body made of the copper alloy material will be hindered.
Particularly, when the material is a copper alloy, if exposed to a high temperature, oxidation progresses from the surface of the material to the inside of the material due to oxygen, a segregation layer is formed by the additional element of the copper alloy (layer due to the oxide of the additional element) at an interface between the formed oxide film of copper and the copper alloy material, and the oxide film of the copper ends up being peeled off together with this segregation layer. This is caused by the large difference in density between the copper oxide film and segregation layer and the copper alloy material and therefore mismatching or incoherency at the interface. As a result, when the semiconductor device is finally completed, a problem arises in the adhesion with the sealing resin.
In order to solve this problem, Japanese Examined Patent Publication (Kokoku) No. 5-19820 proposes a semiconductor device lead frame comprised of a lead frame material made of copper or a copper alloy on which is formed a copper-zinc alloy plating film serving as an under plating of the precious metal plating (silver plating).
In the above proposed lead frame, the zinc of the copper-zinc plating film acts as a barrier against diffusion of the oxygen from the outside and suppress the formation of the segregation layer, whereby a semiconductor device lead frame excellent in adhesion with the sealing resin is obtained.
The above proposed lead frame, however, suffered from the following problems (1) to (4):
(1) When plating a precious metal (plating silver as a representative case) on a copper-zinc alloy plating film in a later step, the zinc of the copper-zinc alloy plating film leaches into the precious metal plating solution and contaminates it, so the precious metal plating film becomes irregular or uneven and, in addition, the adhesion of the precious metal plating film is lowered.
(2) When plating a precious metal (plating silver) on the wire bonding portions, an unnecessary silver plating film is formed up to an inner lead side or back side. When peeling off this unnecessary silver plating film, the zinc of the copper-zinc alloy plating film leaches into the peeling solution, so not only is the original barrier function of the copper-zinc alloy plating film degraded, but also patch-like irregularities or unevenness are formed at the surface left after the peeling and the appearance is remarkably degraded.
(3) In the step of assembling the semiconductor device, an external solder plating film is formed on the outer leads after sealing by the resin. This external solder plating is directly formed on the lead frame material exposed by removal of the copper-zinc alloy plating film. If any of the copper-zinc alloy plating film remains on the surface of the material after the step of removing it, patch-like irregularities or unevenness will form at the solder plating film and the appearance will be degraded and, at the same time, the adhesion of the solder plating film will be lowered.
(4) Without a precious metal plating, outer leads cannot satisfy the requirements of corrosion resistance and moisture resistance if there is any zinc, which is highly corrodible, at the surface. The surface ends up degrading before the step of mounting the semiconductor chip to assemble the semiconductor device. Therefore, this lead frame is not practical.
An object of the present invention is to provide a semiconductor device lead frame excellent in an adhesion with a sealing resin, free from contamination of a precious metal plating solution (particularly a silver plating solution), having a good appearance of the precious metal plating film, excellent in corrosion resistance and moisture resistance, and having a good appearance and adhesion of an outer lead solder plating film, a process of production of the same, a semiconductor device using the lead frame, and a process of production of the same.
To attain the above object, according to a first aspect of the present invention, there is provided a semiconductor device lead frame made of copper or a copper alloy and used for a resin sealing type semiconductor device, comprising:
a lead frame body made of copper or a copper alloy,
a double-layer under plating film formed on the lead frame body and consisting of a lower layer made of zinc or a copper-zinc alloy and an upper layer made of copper having a thickness of 0.02 to 0.4 xcexcm, and
a precious metal plating film formed on at least a wire bonding portion of an inner lead of the copper upper layer of the under plating film.
According to a second aspect of the present invention, there is provided a semiconductor device lead frame made of copper or a copper alloy and used for a resin sealing type semiconductor device, comprising:
a lead frame body made of copper or a copper alloy,
a double-layer under plating film formed on the lead frame body and consisting of a lower layer made of one type of metal selected from silver, tin, iron, cadmium, a copper-nickel alloy, and copper-cadmium and an upper layer made of copper, and
a precious metal plating film formed on at least a wire bonding portion of an inner lead of the copper upper layer of the under plating film.
In the past, due to the heat treatment when mounting a semiconductor chip on a lead frame made of copper or a copper alloy in the step of assembling the semiconductor device, an oxide film was formed at the interface between the copper under plating film and the lead frame material, so the adhesion between the sealing resin and the lead frame was lowered.
In the first aspect of the present invention, provision is made of a double-layer under plating film consisting of a lower layer made of zinc or a copper-zinc alloy and an upper layer made of copper having a thickness of 0.02 to 0.4 xcexcm.
By setting the thickness of the copper upper layer provided on the zinc or the copper-zinc alloy lower layer to at least 0.02 xcexcm, formation of an oxide film at the interface between the under plating film and the lead frame material can be prevented. Namely, in the heat treatment when mounting the semiconductor chip, the copper plating film at the surface layer is oxidized, but after the zinc or the copper-zinc alloy plating film is oxidized, oxygen will no longer reach the copper alloy material of the lead frame body, oxidation of the material will be greatly suppressed, and, as a result, no segregation layer of the additional element of the alloy (layer of the oxide of the alloy element) will be formed, so that the peeling of the oxide film and the interface of the material can be prevented.
Further, the zinc or the copper-zinc alloy plating film of the lower layer is covered by the copper plating film of the upper layer and therefore is not exposed. Due to this, the following effects 1) to 3) are further obtained in addition to the effect described above.
1) When forming a precious metal plating film (particularly a silver plating film) on the under plating film, the zinc will not leach out and contaminate the precious metal plating solution and sufficient adhesion of the precious metal plating film can be secured.
2) Also, when peeling off the unnecessary precious metal plating film on the inner lead side and back side, zinc will not leach out into the peeling solution, so the function of the required locations of the zinc or copper-zinc alloy plating film will be secured, no patch-like irregularities or unevenness will form at the surface at locations where the precious metal plating film is not formed, and a good appearance can be secured.
3) When washing with an alkali to remove the resin flashes, a step performed before the external solder plating of the outer lead, the amphoteric and fast reacting zinc will not leach out into the alkali washing solution, so there is no unevenness of the solder plating film due to abnormal precipitation in the solder plating or heating and a good appearance is secured.
On the other hand, by setting the thickness of the copper upper layer to not more than 0.4 xcexcm, the under plating film can be completely removed by washing by an acid before the external solder plating of the outer leads (after washing by an alkali to remove the resin flashes), therefore no under plating film will remain and lower the appearance and the adhesion of the solder plating film.
This is particularly important for securing a good solder bonding strength. Namely, when soldering the surface of the material of a lead frame made of copper or a copper alloy with tin and lead (Snxe2x80x94Pb), an Sn3Cu4 intermetallic compound is formed at the interface of the material and solder. The resultant bonding structure [Cu/Sn3Cu4/Snxe2x80x94Pb] provides a high bonding strength.
As opposed to this, when zinc remains on the surface of the material, the formation of the Sn3Cu4 intermetallic compound when soldering is obstructed since zinc is interposed between the solder and the material at the locations where the zinc remains. The bonding structure [Cu/Zn/Snxe2x80x94Pb] obtained as a result of this has a low bonding strength at the Cu/Zn interface and the Zn/Snxe2x80x94Pb interface, so the solder bonding strength is lowered.
To remove the under plating film before the outer lead solder plating, the standard practice is to use sulfuric acid or another acid having a concentration of about 10 to 15 v/v % and a pH of about 1. With an acid weaker than this, the removal time becomes extremely long, while with an acid stronger than this, the surface of the outer leads become rough, so neither are practical.
The thickness of the copper upper layer is further desirably set at 0.1 to 0.3 xcexcm by which the, above effects are obtained most stably.
The zinc or zinc alloy lower layer is made thick enough to prevent the oxidation at the interface between the copper or copper alloy material and the under plating film and thin enough to be easily removed by the acid washing before the outer lead solder plating. From this viewpoint, the lower layer can be formed as a strike plating film having a thickness of about 0.001 to 0.5 xcexcm, desirably about 0.01 xcexcm.
In the second aspect of the invention, provision was made of a double-layer under plating film comprised of a lower layer made of one type of metal selected from among silver, tin, iron, cadmium, copper-nickel alloy, and copper-cadmium and an upper layer made of copper
In the second aspect of the invention, the metal of the lower layer has a higher corrosion resistance and oxidation resistance than zinc and does not leach out much into acid, so the thickness of the copper upper layer can be reduced from that in the first aspect of the invention. Generally, the thickness of the copper upper layer is set at 0.1 to 0.3 xcexcm. The lower layer can be formed as a strike plating film having a thickness of about 0.001 to 0.5 xcexcm, desirably about 0.01 xcexcm, from the same viewpoint as the first aspect of the invention.
Further, according to a third aspect of the invention, there is provided a semiconductor device lead frame made of copper or a copper alloy and used for a resin sealing type semiconductor device, comprising:
a lead frame body made of copper or a copper alloy,
an under plating film made of a copper-nickel alloy or a copper-cadmium alloy formed on the lead frame body, and
a precious metal plating film formed on at least a wire bonding portion of an inner lead of the under plating film.
A copper-nickel alloy and a copper-cadmium alloy particularly have a high corrosion resistance and oxidation resistance and do not leach out much into an acid, therefore can be formed as the under plating film without the copper upper layer. Note that when compared with the first aspect of the invention and the second aspect of the invention provided with the under plating film of a double-layer structure, the adhesion of the oxide film formed at the interface with the precious metal plating film is somewhat low, so it is desirable to suitably select the application. In this case as well, the under plating film can be formed as a strike plating film having a thickness of about 0.001 to 0.5 xcexcm, desirably about 0.01 xcexcm, from the same viewpoint as the lower layer of the first aspect of the invention.
The process of production of a semiconductor device lead frame of the first aspect of the present invention comprises:
a step of forming a lead frame body made of copper or a copper alloy,
a step of forming a lower layer plating film made of zinc or a copper-zinc alloy on the lead frame body,
a step of forming an upper layer copper plating film having a thickness of 0.02 to 0.4 xcexcm on the zinc or the copper-zinc alloy lower layer plating film and thereby forming a double-layer under plating film consisting of the lower layer and the upper layer, and
a step of forming a precious metal plating film on at least a wire bonding portion of an inner lead of the upper layer copper plating film.
The thickness of the upper layer copper plating film is most desirably 0.1 xcexcm to 0.3 xcexcm from the same viewpoint as the first aspect of the invention. Also, the zinc or the copper-zinc alloy lower layer can be formed as a strike plating film having a thickness of about 0.001 to 0.5 xcexcm, desirably about 0.01 xcexcm, from the same viewpoint as the first aspect of the invention.
Further, a semiconductor device using a lead frame of the first, second, or third aspect of the invention is formed by mounting a semiconductor chip on a lead frame, electrically connecting the semiconductor chip and the wire bonding portions of said inner leads by bonding wires, and sealing said semiconductor chip by a resin.
Generally, the under plating film on the outer leads of the lead frame is removed (peeled off), and the solder plating film is formed on the exposed outer leads.
Particularly, the process of production of a semiconductor device using a lead frame of the first aspect of the invention comprises:
a step of forming a lead frame body made of copper or a copper alloy,
a step of forming a lower layer plating film made of zinc or a copper-zinc alloy on the lead frame body,
a step of forming an upper layer copper plating film having a thickness of 0.02 to 0.4 xcexcm on the zinc or the copper-zinc alloy lower layer plating film and thereby forming an under plating film consisting of the lower layer and the upper layer,
a step of forming a precious metal plating film on at least a wire bonding portion of an inner lead of the upper layer copper plating film,
a step of mounting a semiconductor chip on a die pad of the lead frame formed with the under plating film and the precious metal plating film,
a step of electrically connecting the semiconductor chip and the wire bonding portions of the inner leads of the lead frame by bonding wires,
a step of sealing the semiconductor chip with a sealing resin,
a step of removing the under plating film consisting of the lower layer and the upper layer from the outer leads of the lead frame, and
a step of forming a solder plating film on the exposed outer leads.